1. Field of the Invention
The present invention relates in general to an apparatus for controlling deceleration of a DC motor, and more particularly to a DC-motor deceleration control apparatus arranged to apply a drive current in the form of pulses to the DC motor.
2. Discussion of Related Art
A conventional control apparatus used to drive and control a DC motor by PWM control employs a so-called “bridge circuit” which is arranged to apply an electric current to the DC motor in opposite directions (forward and reverse directions). This bridge circuit will be described in detail by reference to FIG. 1, which shows an arrangement of a driver device used to drive a carriage drive DC motor 18 (hereinafter referred to simply as “CR motor 18”) which is provided to move a carriage of a printer through an endless belt.
As shown in FIG. 1, the motor driver device includes a first pair of switching elements S1 and S2 disposed in series with each other between positive and negative poles Pv, P0 of an electric power source, and a second pair of switching elements S3 and S4 also disposed in series with each other between the two poles Pv, P0. The first and second pairs of switching elements S1, S2, S3, S4 are connected in parallel to each other. To each of the switching elements S1-S4, there is connected a flywheel diode FD in parallel. The switching elements S1-S4 may be PNP type or NPN type transistors with the flywheel diodes FD connected thereto. However, the switching elements may be field-effect transistors (FET) which incorporate parasitic diodes equivalent in function to the flywheel diodes. In this case, exclusive flywheel diodes are not provided externally of the field-effect transistors.
One of the two terminals of the CR motor 18 is connected between the two switching elements S1 and S2 while the other terminal is connected between the other two switching elements S3, S4. When the switching elements S1 and S4 are turned on while the switching elements S2 and S3 are turned off, an electric current (forward-drive electric current) is applied to the CR motor 18 in a forward-drive direction so that the CR motor 18 is operated in the forward direction. When the switching elements S2 and S3 are turned on while the switching elements S1 and S4 are turned off, on the other hand, an electric current (reverse-drive electric current) is applied to the CR motor 18 in a reverse-drive direction so that the CR motor 18 is operated in the reverse direction.
The CR motor 18 can be operated at a constant speed or smoothly decelerated, in the forward direction, for example, by controlling an average amount of electric current applied to the CR motor 18 by alternately turning on and off the switching elements with pulse signals. To this end, the switching elements S1-S4 are connected to a control portion through a CR-motor driver circuit 31, so that the switching elements are turned on and off under the control of the control portion, to move the carriage at a desired speed or stop the carriage at a desired position.
To stop the carriage, for example, the duty ratio (ON time ratio) of the switching element Si is gradually reduced while the switching element S4 is kept on, as indicated in FIG. 13A. In this case, the electric current in the form of a pulse signal is applied to the CR motor 18, and the duty ratio of the pulse signal is gradually reduced, so that the CR motor 18 can be smoothly stopped at a desired position. When the switching element S1 is in the off state while the switching element S4 is in the on state, the electric current flows in the reverse-drive direction from the negative pole P0 to the positive pole Pv through the switching element S4 and the flywheel diode FD of the switching element S1, so that the CR motor 18 is subjected to a so-called “regenerative brake”.
A regenerative braking force generated by the regenerative brake is relatively large while the CR motor 18 is operating at a relatively high speed, and the regenerative braking force decreases with a decrease in the operating speed of the CR motor 18. Where the duty ratio of the switching element S1 is gradually reduced as described above, a relatively long time is required until the CR motor 18 is brought to a complete stop, and the required stopping distance of the carriage is relatively long, as indicated in FIG. 14A.
In view of the above, it is considered to positively apply an electric current in the form of pulses to the CR motor 18 in the reverse-drive direction during a selected portion of the deceleration period of the CR motor 18 with the switching elements S1 and S4 controlled as described above, by controlling the duty ratio (ON time ratio) of the switching element S3 while the switching element S2 is held on, as indicated in FIG. 13B.
When the electric current is applied to the CR motor 18 in the reverse-drive direction with the switching elements S2 and S3 being controlled as described above, the operating speed of the CR motor 18 is abruptly or rapidly reduced, as indicated in FIG. 14B. Accordingly, the application of the electric current in the reverse-drive direction during the selected portion of the deceleration period as indicated in FIG. 13B makes it difficult to exactly control the position at which the carriage is stopped.